Scaffolding proteins play an important role in cell biology by organizing proteins important for cell signaling, gene transcription, protein targeting and cell polarity. This grant proposal will examine the importance of scaffolding proteins in glomerular podocytes, a cell that is plays an important role in many kidney diseases including diabetes and glomerulonephritis. This proposal focuses on scaffolding proteins containing PDZ domains. PDZ domains bind to the extreme carboxy-terminus of many cell surface proteins controlling the targeting of these cell surface proteins. One group of PDZ domain scaffolding proteins is called Membrane Associated Guanylate Kinase (Maguk) Proteins. Maguk proteins combine a PDZ and SH3 domain with a catalytically inactive guanylate kinase domain. All the domains identified to date in Maguk proteins are involved in protein-protein interactions and our work has focused on these interactions. Pals1 is a tight junction Maguk protein that exists in a complex with several proteins including Crumbs and Patj. Drosophila homologues of Pals1, Patj and Crumbs are crucial for proper epithelial polarity in Drosophila and their mammalian counterparts likely fulfill a similar role in mammalian epithelia. In podocytes Pals1 is localized to the slit diaphragm. Another tight junction protein we have identified in the podocyte is a Maguk protein known as Magi-1. Magi-1 interacts with two actin binding podocyte proteins, synaptopodin and alpha actinin-4. This proposal will further examine the role of Magi-1 and Pals1 in podocyte function using biochemical, immunologic and genetic approaches. We hypothesize that these proteins are crucial for the proper organization of the slit diaphragm and overall polarity of glomerular podocytes. The proposal will utilize the ability of the transgenic mouse core of the University of Michigan O'Brien Renal Center to specifically alter gene expression in the podocyte in order to examine the role of Pals1 and Magi-1 in glomerular function.